Generally speaking, pan granulation has been proposed in the art many times previously and with significant variations.
One of the latest issued patents in the art to which the present invention relates is U.S. Pat. No. 5,460,765, issued to Derdall et al., Oct. 24, 1995. The reference teaches a process for pan granulating a particulate material. Based on the teachings of the Derdall et al. reference, a final particle size distribution that is achievable by practicing the invention is between about -5 mesh to about +10 mesh. In order to initiate the process, the Derdall et al. process is limited to the introduction of a seeding material typically between about -14 mesh and +28 mesh. This is required in order to control the granule growth and as indicated in the Derdall et al. disclosure, seed minimizes mutual agglomeration and results in high yields being obtained. The Derdall et al. reference further indicates that the proper sizing of the seed is fundamental to the operation of the process for granulation in order to have product yields exceed 90%. Reference is made in the disclosure that a seed core in the range of -14 mesh to +35 mesh is required in order to achieve a steady state and maintain uniform size distribution of between -8 mesh to +6 mesh.
One of the greatest limitations existing in the granulation art is centered on the fact that known processes require a seeding agent in order to achieve the proper conditions for material accretion to result in a pellet or granule. By making use of a seed, the resulting granule is adversely affected in two key properties; roundness and cross-sectional uniformity. Typically, seeding material is not round and as the precursor particle, the result is irregular initial feedstock accretion which, in turn, forms an out-of-round particle upon which further material accretes. A further detriment from this results in terms of nonuniform particle density.
Methodology is required for synthesizing a granule in the absence of seed material and which is round, tightly packed with a uniform homogeneous cross-section and capable of eliminating hazards associated with fertilizer granule production.
The Derdall et al. process, although a meritorious procedure, did not recognize the limitations of employing a seeding agent or the need for controlling the dust generated during granulation which not only creates an unhealthy environment for workers, but more seriously, results in a potentially explosive environment. This is evident from the teachings of Derdall et al., particularly at column 3, beginning at line 24, wherein it is stated:
"It may be more difficult to keep the granulation steady or stable with fine seed, such as -35 mesh." PA1 "Fine seed sizes can be used, such as +35 mesh, but a point is reached where over-seeding or nucleation occurs easily and causes the final product yield to drop down." PA1 It is also indicated at column 3, beginning at line 45 that: PA1 "Seed material in the range of 20 mesh is the best single point for each of control and uniformity of product size distribution . . . " PA1 "Seed of large size forms granules of very poor strength." PA1 providing an initial feedstock having a uniform size distribution of about 99.9% particle size of -150 mesh and of said 99.9% particle size of -150 mesh about 90% comprising a particle size of -200 mesh; PA1 providing a nucleating material in a size distribution of 35 mesh to 150 mesh; PA1 providing a binder solution; PA1 contacting the binder and the feedstock in a pan granulator; and PA1 forming fertilizer granules directly in the pan granulator in the absence of a seeding agent. PA1 i) an enhanced market value of the formed product; PA1 ii) the formation of a more user friendly product; PA1 iii) significantly improved product handleability; PA1 iv) a significantly improved uniformity index: PA1 v) no product loss or waste. PA1 vi) uniform cross-section; PA1 vii) tightly packed feedstock; PA1 viii) absence of a seed or crystal core; PA1 ix) increased break strength relative to the prior art; PA1 x) material homogeneity throughout the granule; and PA1 xi) greater quantity of feedstock material per granule
The difficulty to which the Derdall et al. disclosure alludes is directed to cycling which is an inherent problem with pan granulation processes. If the size distribution of the seeding agent is not constant, then the process will not stabilize and effectively "cycles" as is known to those skilled in this art. The result of this is that larger formed granules on the pan effectively destroy the smaller particles. This, of course, defeats the purpose of the pan granulation to generate particles.
Furthermore, at line 36 in column 3, the disclosure indicates that:
As is known, the larger the mesh numerical value the smaller the micron size of the respond to the stated micron sizes:
Approximate Mesh Size Micron Size 12 1680 16 1190 20 840 30 590 40 420 100 149 200 74
Based on the teachings of the Derdall et al. disclosure, mesh sizes greater than +35 cause potential nucleation problems and result in a final product yield to decrease. With the technology disclosed infra, it has been found that by using a fine powder of between -35 mesh to +150 mesh, that a superior quality product can be formed in a high yield and typically in the range of greater than 90% yield. When the above passage regarding Derdall et al. is considered, it is clear that Derdall et al. effectively contradict what the technology set forth herein has found to be particularly successful.
In the present application the size distribution of the nucleating material is between -35 mesh and +150 mesh which corresponds to micron size less than 590 .mu.m and 105 .mu.m, respectively. Nowhere in the prior is a powdered nucleating agent in this size distribution disclosed for purpose of forming a uniform granule in the size distribution of, for example, 3 mesh to about 10 mesh. Advantages have been ascribed to this process and one of the most attracttive advantages is that the granule or pellet has an enormous break strenght and a uniform cross-section, It has been found by practicing the present invention, that break strenghts in the range of 1 to 4 kg or greater have been achieved.
In the Derdall et al. disclosure, at column 3, beginning at line 33 it is stated:
If one considers these teachings in light of the size of the nucleating agent provided herein, the admissions made in the Derdall et al. disclosure would clearly go against the appeal of using a seeding agent in the size range as clearly taught by Derdall et al. The instruction in Derdall et al. indicates an ideal seeding agent size is 20 mesh (supra); the instant application uses a powder having a particle size between 75-750% smaller than Derdall et al. and yet achieve very desirable results.
In Statutory Invention Registration H1070, authored by Harrison et al., Jul. 7, 1992, a method for granulating potash materials is disclosed. The process involves the conversion of particulate potassium sulfate or potassium chloride by agglomeration using a conventional rotary drum granulator, pan granulator or other conventional granulating device.
In the disclosure of this document, there are no specific teachings regarding the elimination of a seeding agent, feedstock size or other important factors related to process control in order to generate superior quality granules having commercial viability. Further, the process clearly is an agglomeration process. It is known that agglomeration typically involves the aggregation of colloidal particles suspended in a liquid into clusters or flocs. These clusters or flocs have varying degrees of interstice and are loosely bound (Hawley's Condensed Chemical Dictionary, eleventh edition, 1987).
It would be desirable if it were possible to achieve a high yield granular product within specified mesh sizes which does not require the use of a seed in the process.
The present invention is directed to satisfying the limitations in the art and more particularly those limitations in the Derdall et al. reference.